Many thermoplastic and thermosetting polymers have excellent rigidity, dimensional stability, transparency, and impact resistance, but unfortunately have poor abrasion resistance. Consequently, structures formed from these materials are susceptible to scratches, abrasion, and similar damage. To protect these structures from physical damage, a tough, abrasion resistant “hardcoat” layer may be coated onto the structure. Such coatings are desirably transparent and are typically present as very thin coatings.
In addition to abrasion resistance, it is often desirable that the hardcoat provide resistance to marking and/or easy removal (i.e., smudge resistance), by fingerprints, markers, spray paint, and the like. Smudge resistant nanocomposite hardcoats are of particular interest for optical display applications requiring the protection of a plastic substrate from marring or scratching while also providing an easily cleaned surface.
One of the most effective technical approaches to hardcoat materials has been to incorporate inorganic nanometer scale particles into a binder precursor resin matrix. The surface of the particles may be functionalized with a variety of coupling agents to improve compatibility with and/or bonding to the binder resin to improve the abrasion resistance of the hardcoat.
Approaches to smudge resistant nanocomposite hardcoats have included the incorporation of fluorinated organosilane coupling agents, fluorinated monomers, and/or fluorinated surfactants into the formulations. In each case, the fluorinated material is incorporated into the bulk of hardcoat construction. Fluorinated materials that are not situated on the nanocomposite hardcoat's exposed outer surface do not provide effective smudge resistant properties, and may have a deleterious effect on abrasion resistance, adhesion of the hardcoat to the substrate, and add unnecessary cost to the hardcoat formulation. In addition, fluorinated materials that are not covalently bound to the nanocomposite hardcoat may be removed during washing and their smudge resistance degraded.
Similarly, a fluorinated organosilane coating applied to the surface of a hardcoat may not effectively bond to the hardcoat, and lose smudge resistance upon repeated chemical or mechanical insult to the coating due to a lack of covalent bonding between the smudge resistant layer and the hardcoat layer. Thus, there exists a need for nanocomposite hardcoat constructions having improved durability of smudge resistance.